For producing a small size and a mid-size hot seamless tube, a tube-making method by a mandrel mill rolling process is applied.
FIG. 1 is a diagram explaining the outline of the tube-making step by a mandrel mill rolling process. In this tube-making method, a solid round billet 1 as a work material to be rolled being heated to a preset temperature is subjected to a piercing process by a piercing-rolling mill 3 (so-called piercer mill) where the centerline portion thereof is pierced, thus a hollow tube stock 2 is made. Subsequently, the hollow tube stock 2 thus obtained is moved to a mandrel mill 4 where an elongation-rolling process is applied.
In a mandrel mill 4, a plurality of reduction roll pairs 6 to be used for rolling the hollow tube stock 2 where each roll of the pair is set as opposed to each other with respect to a pass line are disposed, and the hollow tube stock 2 is subjected to an elongation-rolling process in which said stock is rolled and elongated by means of the reduction rolls 6 for pressing the outer surface of said tube stock coupled with the mandrel bar 5 inserted into said tube stock. In general, the reduction rolls 6 are housed in the roll stand and each pair of reduction rolls 6 in roll stands lying side-by-side are set with 90° of phase angle, thereby the hollow tube stock 2 is rolled while altering the circumferential direction of reduction rolling by 90° in phase angle at every roll stand.
The mandrel bar 5 to be used for the mandrel mill rolling process is conventionally manufactured from the round bar made of Hot Working Tool Steel such as JIS SKD6 or SKD61. Further, in order to secure the toughness and crack resistance, it is common that the entire body of mandrel bar having smooth surface by polishing is quenched and tempered, thereby the surface hardness is controlled to be HV350–450 or so and the mill scale film is formed over the surface of the mandrel bar.
In the mandrel mill rolling process, in order to avoid the seizure between the hollow tube stock 2 and the mandrel bar 5, a water-soluble lubricant primarily composed of the solid lubricant is coated and dried on the surface of mandrel bar 5 to preform the film of solid lubricant prior to performing elongation-rolling. Besides, when it is necessary to enhance the effect of the lubrication, the solid lubricant is provided onto the inner surface of the hollow tube stock where said lubricant is melted by the heat retained within the hollow tube stock 2, thereby the liquid lubricant film can be preformed.
With the liquid lubricant film thus preformed, the friction force to be generated between the inner surface of the hollow tube stock 2 and the surface of the mandrel bar 5 when an elongation-rolling process is applied and/or the mandrel bar 5 is withdrawn can be reduced, thus the seizure of the hollow tube stock as well as the wear of the mandrel bar 5 can be prevented.
However, as there exists the persistent sliding friction at the interface between the inner surface of the hollow tube stock 2 and the surface of the mandrel bar 5 during an elongation-rolling process, it is hard to maintain a perfect lubrication at the interface between them. Thus, while using the mandrel bar repeatedly, the surface defects such as wear, seizure, rough surface or crack inevitably develop, resulting in ending the service life thereof. Meanwhile, the mandrel bar becoming out of service life is reused as the one with a smaller diameter after machining the outer surface.
In this regard, the tool cost relative to the production cost of a hot seamless tube by a mandrel mill rolling process, especially the expense to be spent for the mandrel bar, is very high. Therefore, for the purpose of reducing the production cost of a hot seamless tube, it has been studied to improve the surface condition which serves to suppress the occurrence of surface defects on the mandrel bar and to extend the service life thereof.
For example, in Japanese Patent Application Publication No. 63-20105 (hereinafter, referred to as a patent document 1), there is proposed a surface conditioning method for reducing the friction coefficient in the mandrel mill rolling process and enhancing the tightness of the mill scale film, wherein the dimples with maximum depth 50 μm are provided on the surface of the mandrel bar at the rate of two or more dimples per 1 mm in length.
Also in Japanese Patent Application Publication No. 04-284905 (hereinafter, referred to as a patent document 2), there is proposed a surface conditioning method for a mandrel bar for use in rolling a hot seamless tube, wherein the surface of the mandrel bar itself is polished in circumferential direction and then the finishing polishing is conducted so as to ensure 4–12 μm of surface roughness Ra in longitudinal direction. Further, in Japanese Patent Application Publication No. 08-164404 (hereinafter, referred to as a patent document 3), there is proposed a mandrel bar for use in rolling a hot seamless tube, specifying the surface roughness in circumferential direction to be 1.0–4.0 μm in centerline average roughness (Ra).
The mandrel bar and the surface conditioning method thereof in above patent documents 1–3 are intended to enhance the tightness of the mill scale film, taken for granted that the mill scale film is formed on the surface.
Thus, an effect regarding the tightness of the mill scale film can be appreciated to some extent in case of the mandrel bar according to the patent documents 1–3. Nonetheless, even if the surface conditioning as proposed were conducted on the mandrel bar, the surface layer of the mandrel bar should be exposed to the high temperature of 500–600° C. during an elongation-rolling process so that the oxidization-decarburization on the surface layer of the bar should take place and cause the softening of the surface layer. The mandrel bar with such a softened surface layer causes the seizure even if the mill scale film were formed. Therefore, it is not possible to fully expect the extension of the service life of the mandrel bar by merely forming the mill scale film thereon.
As described above, there exists a certain limit in extending the service life of the mandrel bar in case of forming the mill scale film. So, a mandrel bar on which a Cr-plated hard film is formed to enhance the wear resistance is recently utilized. Namely, by forming a thick Cr-plated layer of as much as 50 μm, the direct contact of the oxygen in air with the surface of the substrate is eliminated, thus preventing the oxidation-decarburization.
FIG. 2 is a diagram comparing the service life of the mandrel bar in case that a stainless steel is subjected to a mandrel mill rolling process. The service life of the Cr-plated mandrel bar is normalized to the service life of the mandrel bar having the mill scale film formed, i.e. the service life of the mandrel bar having the mill scale film formed is rated as 1. For a reference, the grades of the stainless steel that are used for tube-making are SUS420J1 and the like.
The service life of the Cr-plated mandrel bar is remarkably extended in comparison with that having the mill scale film formed, whereas it is extended by a factor of five in average, although depending on the tube size to be made, the material grade, and especially the amount of thickness reduction.
Normally, to employ a Cr-plated mandrel bar, it becomes necessary to install a plating equipment as an initial investment, but a running cost onwards becomes equivalent to that for the conventional mandrel bar having the mill scale film formed. Thus, the Cr-plated mandrel bar becomes a primary target in respect of the campaign of extending the service life which serving for reduction of the production cost.
In this aspect, several proposals for extending the service life of a Cr-plated mandrel bar are offered further. For example, in Japanese Patent Application Publication No. 08-71618 (hereinafter, referred to as a patent document 4), there is proposed a mandrel bar having a Cr-plated film with centerline average roughness (Ra) of 1.0–4.0 μm in axial direction, and in Japanese Patent Application Publication No. 2000-246312 (hereinafter, referred to as a patent document 5), there is proposed a mandrel bar having a Cr-plated film with centerline average roughness (Ra) of 0.1 μm or more but below 1.0 μm in axial direction.
Further, in Japanese Patent Application Publication No. 2001-1016 (hereinafter, referred to as a patent document 6), there is made a proposal on a mandrel bar having a Cr-plated film with a thickness of 60–200 μm, and in Japanese Patent Application Publication No. 2000-351007 (hereinafter, referred to as a patent document 7), there is made a proposal on a mandrel bar wherein the waviness of the surface in length-wise direction is specified.
In the mandrel bar proposed in above patent documents 4–7, taken granted that Cr-plating is performed, the service life can be extended as expected. However, the demand for reducing further the production cost of a hot seamless tube in a mandrel mill rolling process is so strong, and in particular the tool cost reduction is mostly concerned among others. In such a circumstance, a further dramatic improvement in respect of the extension of the service life of the mandrel bar is demanded.